Lie detection devices, i.e. devices which measure psychological stress as an indicator of deception, are commonly used in the fields of law enforcement and the military, and occasionally in the private sector. The oldest type of lie detection device, known as the polygraph, measures changes in a person's body associated with the stress of deception, including alterations in heart rate, breathing, and electrodermal activity. In a polygraph examination, the subject is required to be “wired” to the examination equipment in order to record the various physiological changes. Present day polygraph devices are now computer implemented. However, the basic technology has changed very little.
Later advances in the field of lie detection technology have focused on the detection of stress in human speech in an attempt to produce a portable lie detection device which could be used “in the field.” Voice stress detection devices provide an advantage over the traditional polygraph in that they do not require that the subject be “wired,” and are thus a non-invasive means of truth detection.
The scientific basis was provided by the discovery of vocal chord micro-tremors in the 1950's. In 1971, Olof Lippold, a researcher at University College, London, published the results of research he had begun in 1957 in the magazine Scientific American. Lippold's article, entitled “Physiological Tremor,” explored the initial discovery by Martin Halliday and Joe Redfeam of the phenomena a decade previously at the National Hospital in London. Halliday and Redfeam discovered that the contraction of a voluntary muscle is accompanied by tremors of the muscle in the form of minute oscillations. According to Lippold, all muscles in the body, including the vocal chords, vibrate in the 8 to 12 Hz range. Lippold explained that the tremor is believed to be a function of the signals to and from motor neurons and that it is analogous to a self-adjusting closed-loop servo system, and that muscles tighten and loosen as they seek to maintain a constant tension. In moments of stress, the body prepares for fight or flight by increasing the readiness of its muscles to spring into action. Their vibration increases from the relaxed 8 to 9 Hz to the stressful 11 to 12 Hz range. A method of detecting psychological stress by evaluating physiological changes in the human voice is disclosed in U.S. Pat. No. 3,971,034 issued to Bell, the contents of which are herein incorporated by reference.
Voice stress patterns were first introduced as a method of measuring psychological stress beginning with the Psychological Stress Evaluator designed by W. H. Ford, A. D. Bell, and C. R McQuiston in 1971. The instrument, the PSE, recorded tracings on a strip chart utilizing EKG paper and a heated stylus. Each response produced movements of the stylus that made vertical lines on the paper which were recognizable as either stressful or non-stressful patterns. This determination was made according to the length, and to some degree, the shape of the pattern. However, very little effort was made to establish exact patterns or to provide a formula for determining deception, lending considerable confusion to the evaluation process and creating many errors.
Generally speaking, evaluated voice patterns come in all shapes and sizes. These patterns include variations of male, female, regional and international linguistic variations, etc. Regardless, human voice patterns share many characteristics and a similar stress-related trait, namely the tendency of a stress pattern to form a roughly rectangular pattern. This tendency is commonly know as “blocking.”
Referring now to FIG. 1A, a voice pattern is shown for a non-stressed, truthful vocal utterance, which, in this example, has a roughly triangular envelope. FIG. 1B shows the same utterance spoken as a non-truth, i.e. under psychological stress. It can be seen that the envelope of the pattern is approximately rectangular.
In prior art voice stress analysis systems, the determination of psychological stress is made empirically by a human examiner. A problem with the prior art systems is that since the judgment is made purely by visual observation, the personal bias of the examiner may enter into the scoring of the voice pattern. Also, since the results are based on individual human judgment, they can be influenced by, for example, the quality of training and the cumulative field experience of the individual examiner. This will naturally create inconsistencies in the scoring between individual examiners.
Thus, what is needed to solve the drawbacks of the prior art voice analysis systems is a computer-implemented scoring system which utilizes an algorithm to analyze the voice sample and generate a score. The computer-generated score can be used as a consistent guideline for the examiner as a means to provide non-biased, consistent scoring of psychological stress in a human subject.